Microsomal Δ5 desaturation of eicosa-8,11,14-trienoic acid is activated by a cytosolic fraction.

Δ5 Desaturation of eicosa-8,11,14-trienoic acid to arachidonic acid was studied in rat liver microsomes. It was shown that Δ5 desaturation of fatty acids in vitro requires the participation of a peripheral component of cytosolic origin. Desaturation of 20∶3n-6 to 20∶4n-6 decreases in washed microsomes as they lose an adsorbed cytosolic fraction (CF), but the enzymatic activity can be recovered as a function of CF concentration in the incubation medium. Albumin does not substitute for CF. Δ5 Desaturation of 20∶3n-6 is inhibited by arachidonic acid by a product inhibition effect, but CF prevents retroinhibition of Δ5-desaturase by 20∶4n-6. This ability of CF is eliminated by preincubation of CF with 20∶4n-6, but not with γ-18∶3n-6, the product of Δ6 desaturation of 18∶2n-6, thus indicating that CF impairs the retroinhibitory effect of arachidonic acid on Δ5-desaturase in a specific manner. Δ6 Desaturation of linoleic acid to γ-18∶3n-6 is also activated by CF and retroinhibited by γ-18∶3n-6. CF activity on Δ6 desaturation is retained after preincubation with 20∶4n-6, but it is lost after preincubation with γ-18∶3n-6. Activation of Δ6-desaturase by CF is associated with the removal of the reaction product in a specific manner. Chromatography of CF by Sephacryl S-200 separates two major subfractions which show different efficiency in reactivating Δ5- and Δ6-desaturase activities in washed microsomes. Therefore, CF may contain subfractions that can prevent Δ5- and δ6-desaturase retroinhibition by apparently binding their respective reaction products specifically.